The present invention relates to an image forming apparatus, such as an electrophotographic copying apparatus or an electrophotographic-type printer, which is capable of detecting a jam of copy paper on which an image is to be transferred.
A copy paper jam detection device in an image forming apparatus such as an electrophotographic copying apparatus generally includes one or more detection switches disposed in the copy paper delivery passage and a timer mechanism. Such a device employs a method in which a paper jam is judged to have happened if the time during which each detection switch is ON, or the time during which each detection switch is changed in status from ON to OFF or from OFF to ON, is longer than a period of time preset in the timer mechanism (See, for example, the Japanese Utility Model Publication Examined No. 7956/1980).
In view of the nature of such a detection device, time setting has been made based on the copy paper of a maximum length allowed to be used in the image forming apparatus. Accordingly, while the copy paper of the maximum length is being used, a paper jam, if happened, may be smoothly detected. However, this method has disadvantageously detected, with delay or error, a paper jam happened while, for example, the copy paper of a minimum size is being used.
To overcome the defect above-mentioned, there has been proposed a jam detection method in which a period of time to be set in the timer mechanism may be changed, as necessary, according to the length of copy paper to be used, as disclosed by the Japanese Laid-Open Patent Publication No. 139765/1982 or No. 131650/1982.
According to the method proposed by the Publications above-mentioned, a period of time to be set in a timer may be changed according to the length of paper to be used. Thus, a paper jam may be detected securely and rapidly. However, likewise a conventional prior art, this method includes room for further improvements in that the place where a paper jam has happened cannot be detected accurately dependent on the sizes of copy paper to be used.
More specifically, where a jam has been detected because one detection switch was not turned ON or OFF as it would be normally done, within a period of time preset in the timer, it is difficult to judge whether a major portion of the jammed paper is present upstream or downstream of the detection switch above-mentioned.
It is an object of the present invention is to provide an image forming apparatus having a function capable of judging the exact place where a paper jam has happened even if the size of copy paper changes, so as to inform the exact jam place to the operator.